1. Field of the Invention
The present invention relates to a hybrid catalyst system containing a mixed metal alkoxide Ziegler-Natta portion and a cycloalkadienyl portion, which is useful for producing polyolefins including broad molecular weight and bimodal polyolefins. The invention also relates to methods of making the hybrid catalyst, and its use in making polyolefins having a bimodal molecular weight distribution.
2. Description of Related Art
For certain applications of polyethylene, toughness, strength and environmental stress cracking resistance are important considerations. These properties are enhanced when the polyethylene is of high molecular weight. However, as the molecular weight of the polymer increases, the processability of the resin usually decreases. By providing a polymer with a broad or bimodal molecular weight distribution, the properties characteristic of high molecular weight resins are retained and processability, particularly extrudability, is improved.
Bimodal molecular weight distribution of a polyolefin indicates that the polyolefin resin comprises two components of different average molecular weight, and implicitly requires a relatively higher molecular weight component and low molecular weight component. A number of approaches have been proposed to produce polyolefin resins with broad or bimodal molecular weight distributions. One is post-reactor or melt blending, in which polyolefins of at least two different molecular weights are blended together before or during processing. U.S. Pat. No. 4,461,873 discloses a method of physically blending two different polymers to produce a bimodal polymeric blend. These physically produced blends, however, usually contain high gel levels, and consequently, they are not used in film applications and other resin applications because of deleterious product appearance due to those gels. In addition, this procedure of physically blending resins suffers from the requirement for complete homogenization and attendant high cost.
A second approach to making bimodal polymers is the use of multistage reactors. Such a process relies on a two (or more) reactor set up, whereby in one reactor, one of the two components of the bimodal blend is produced under a certain set of conditions, and then transferred to a second reactor, where a second component is produced with a different molecular weight, under a different set of conditions from those in the first reactor. These bimodal polyolefins are capable of solving the above-mentioned problem associated with gels, but there are obvious process efficiency and capital cost concerns when multiple reactors are utilized. In addition, it is difficult to avoid producing polyolefin particles that have not incorporated a low molecular weight species, particularly, when the high molecular weight component is produced in the first reactor.
A third and more desirable strategy is direct production of a polyolefin having a broad or bimodal molecular weight distribution by use of a catalyst mixture in a single reactor. In fact, Scott, Alex, xe2x80x9cZiegler-Natta Fends off Metallocene Challenge,xe2x80x9d Chemical Week, pg. 32 (May 5, 1999) states that one xe2x80x9cof the holy grails [of polyolefin research] is getting bimodal performance in one reactor for PE and PPxe2x80x9d (quoting Chem Systems consultant Roger Green). The art recently has attempted to solve the aforementioned problems by using two different catalysts in a single reactor to produce a polyolefin product having a broad molecular weight distribution, or bimodal molecular weight distribution. Such a process is reported to provide component resin portions of the molecular weight distribution system simultaneously in situ, the resin particles being mixed on the subparticle level. For example, U.S. Pat. Nos. 4,530,914 and 4,935,474 to Ewen relate to broad molecular weight distribution polyolefins prepared by polymerizing ethylene or higher alpha-olefins in the presence of a catalyst system comprising two or more metallocenes each having different propagation and termination rate constants and aluminoxane. Similarly, U.S. Pat. No. 4,937,299 to Ewen relates to the production of polyolefin reactor blends in a single polymerization process using a catalyst system comprising two or more metallocenes having different reactivity ratios for the monomers being polymerized.
It is known that metallocenes may be affixed to a support to simulate an insoluble catalyst. U.S. Pat. No. 4,808,561 discloses reacting a metallocene with an aluminoxane and forming a reaction product in the presence of a support. The support is a porous material like talc, inorganic oxides such as Group IIA, IIIA IVA OR IVB metal oxides like silica, alumina, silica-alumina, magnesia, titania, zirconia and mixtures thereof, and resinous material such as polyolefins like finely divided polyethylene. The metallocenes and aluminoxanes are deposited on the dehydrated support material.
An advantage of a homogeneous (metallocene) catalyst system is the very high activity of the catalyst and the narrow molecular weight distribution of the polymer produced with a metallocene catalyst system. The metallocene catalysts suffer from a disadvantage in that the ratio of alumoxane cocatalyst to metallocene is high. In addition, the polymers produced using metallocene catalysts often are difficult to process and lack a number of desirable physical properties due to the single homogeneous polymerization reaction site. Moreover, these catalyst are limited in that they are single site catalysts, and consequently, produce polymer having very narrow molecular weight distribution.
Heterogeneous catalyst systems also are well known, and typically are used to prepare polymers having broad molecular weight distribution. The multiple (e.g., heterogeneous) active sites generate a number of different polymer particles of varying length and molecular weight. These heterogeneous catalyst systems typically are referred to as Ziegler-Natta catalysts. The disadvantage of many Ziegler-Natta catalysts is that it is difficult to control the physical properties of the resulting polymer, and the activity typically is much lower than the activity of the metallocene catalysts. Ziegler-Natta catalyst alone are not capable of making satisfactory polyolefins having a bimodal molecular weight distribution, and metallocene catalysts containing cycloalkadienyl groups supported on silica or aluminum alone are not capable of making satisfactory polyolefins having a broad molecular weight distribution.
The art recently has recognized a method of making bimodal resin by using a mixed catalyst system containing Ziegler-Natta and metallocene catalyst components. These mixed catalyst systems typically comprise a combination of a heterogeneous Ziegler-Natta catalyst and a homogenous metallocene catalyst. These mixed systems can be used to prepare polyolefins having broad molecular weight distribution or bimodal polyolefins, and they provide a means to control the molecular weight distribution and polydispersity of the polyolefin.
W.O Pat. 9513871, and U.S. Pat. No. 5,539,076 disclose a mixed metallocene/non-metallocene catalyst system to produce a specific bimodal, high density copolymer. The catalyst system disclosed therein is supported on an inorganic support. Other documents disclosing mixed Ziegler-Natta/metallocene catalyst on a support such as silica, alumina, magnesium-chloride and the like include, W.O. Pat. 9802245, U.S. Pat. No. 5183867, E.P Pat.0676418A1, EP 717755B1, U.S. Pat. No. 5747405, E.P. Pat. 0705848A2, U.S. Pat. No. 4,659,685, U.S. Pat. No. 5,395,810, E.P. Pat. 0747402A1, U.S. Pat. No. 5,266,544, and W.O. 9613532, the disclosures of which are incorporated herein by reference in their entirety.
Supported Ziegler-Natta and metallocene systems suffer from many drawbacks, one of which is an attendant loss of activity due to the bulky support material. Delivery of liquid, unsupported catalysts to a gas phase reactor was first described in Brady et al., U.S. Pat. No. 5,317,036, the disclosure of which is incorporated herein by reference in its entirety. Brady recognized disadvantages of supported catalysts including, inter alia, the presence of ash, or residual support material in the polymer which increases the impurity level of the polymer, and a deleterious effect on catalyst activity because not all of the available surface area of the catalyst comes into contact with the reactants. Brady further described a number of advantages attributable to delivering a catalyst to the gas phase reactor in liquid form. Brady did not appreciate, however, that a self-supported mixed Ziegler-Natta/metallocene catalyst could be used to form a polyolefin in a single reactor having a broad molecular weight distribution or a bimodal molecular weight distribution.
Another problem associated with the prior art supported mixed catalysts is that the supported catalysts often had activities lower than the activity of the homogeneous catalyst alone. Finally, it is difficult to specifically tailor the properties of the resulting polyolefin using supported mixed catalyst systems.
The prior art mixed supported catalysts also produced polymer, albeit in a single reactor, that essentially contained high molecular weight granules and low molecular weight granules. The problems discussed above that are associated with blending two different polymer particles, are also present in these systems. Moreover, producing different granules of polymers in a single reactor leads to poor reactor control, poor morphology of the resulting polymers difficulties in compounding and difficulties in pelleting the resultant polymer. Finally, it is difficult to ensure adequate mixing of the two polymer components which raises a number of quality control issues.
Coordination complexes of Group IVB metals, xcfx80-bonded ligands and heteroallyl moieties are known as useful olefin polymerization catalysts, and are described in Reichle, et al., U.S. Pat. No. 5,527,752, the disclosure of which is incorporated by reference herein in its entirety. Simply mixing an organocyclic moiety such as indene with a magnesium/zirconium ethoxide, as taught in Reichle, does not produce a catalyst capable of producing polyolefins having a broad MWD. Reacting an organocyclic moiety such as indenylzirconiumtris(pivalate) with magnesium ethoxide required strenuous reaction conditions (a basic solution in hot chlorobenzene), and it did not produce a desirable catalyst, presumably because the indenyl moiety was stripped off of the zirconium. It was heretofore thought not possible to coordinate a complex such as those disclosed in Reichle with a zirconium-containing component to produce a catalyst capable of making a broad MWD polyolefin.
Tajima, et al., U.S. Pat. No. 5,387,567 discloses a method of treating a soluble zirconium complex with an organocyclic moiety (Cp) to produce a catalyst component. The disclosure of Tajima is incorporated by reference herein in its entirety. The catalyst components described in Tajima remain in solution requiring a solution-phase polymerization, and if used in a gas phase polymerization, would require a support such as silica, and the like. The disadvantages of supported catalysts are mentioned above. Disadvantages of a solution catalyst system include difficulties in maintaining the activity of the catalyst over extended periods of time, and inefficiencies in shipping and in handling which typically require manufacture of the catalyst component on-site or in-line with the polymerization process. In addition, the activity of the catalysts described in Tajima is low thereby requiring significant amounts of catalyst, and possible post polymerization removal of catalyst residue.
There exists a need to maximize the benefits of each individual catalyst system (i.e., Ziegler-Natta and metallocene) without suffering a penalty in terms of activity of the catalyst components, and without suffering from the poor reactor control and poor product quality control discussed above. There also exists a need to produce bimodal products having excellent product strength and processability. There also exists a need to develop a catalyst to produce such bimodal polyolefins without suffering from the above-noted problems. In addition, there exists a need to develop catalysts capable of making polyolefins having a broad molecular weight distribution. It also would be desirable to produce polymer granules in a single reactor whereby the granules contain a high molecular weight component and a low molecular weight component.
It is therefore a feature of the present invention to provide a catalyst system that is capable of producing a polyolefin with a broad molecular weight distribution, and to provide a catalyst system that is capable of producing a polyolefin having a bimodal molecular weight distribution in a single reactor. It is an additional feature of the invention to provide a catalyst, a method of making the catalyst, a method of making a polyolefin having a broad molecular weight distribution, and a method of making a bimodal polyolefin using the catalyst that does not suffer from the drawbacks mentioned above. It is yet another feature of the invention to provide a catalyst system that is capable of producing polyolefin granules that contain a high molecular weight component and a low molecular weight component.
In accordance with these and other features of the present invention, there is provided a solid catalyst component for the polymerization of olefin monomers comprising: (i) a Ziegler-Natta catalyst component comprising at least one group IVB metal-containing alkoxide or aryloxide; and (ii) Cp, where Cp is a cycloalkadienyl group having from 3-30 carbon atoms.
In accordance with an additional feature of the present invention, there is provided a method of making a solid catalyst component comprising reacting: (i) a Ziegler-Natta catalyst component comprising at least one group IVB metal-containing alkoxide or aryloxide; and (ii) a Cp-containing complex in a suitable solvent to produce a mixture containing solid catalyst component, and then removing the solid catalyst component from the mixture.
In accordance with yet another feature of the present invention, there is provided a method of making a polyolefin, preferably a broad molecular weight polyolefin and/or a polyolefin having a bimodal molecular weight distribution, comprising contacting, under polymerization conditions, at least one olefin monomer with a solid catalyst component comprising: (i) a Ziegler-Natta catalyst component comprising at least one group IVB metal-containing alkoxide or aryloxide; and (ii) Cp, where Cp is a cycloalkadienyl group having from 3-30 carbon atoms. These and other features of the invention readily apparent to those skilled in the art can be achieved by reference to the detailed description that follows.